1. Field of the Invention
The present invention relates to a quadrature signal generator applied to a ZERO-IF converter or image rejection converter of a broadcast receiver, and more particularly to a quadrature signal generator for generating an in-phase (hereinafter, referred to as an “I signal”) and a quadrature-phase signal (hereinafter, referred to as a “Q signal”) while having a feedback type frequency doubler capable of making the Q signal have the same frequency as a differential oscillating frequency, using a feedback control system, thereby securing more reliable symmetric characteristics.
2. Description of the Related Art
Generally, the ZERO-IF converter and image rejection converter of a satellite broadcast receiver use a quadrature modulator for generating modulated I and Q signals. Such a quadrature modulator is illustrated in FIG. 1.
FIG. 1 is a schematic diagram illustrating a general quadrature modulator. Referring to FIG. 1, the quadrature modulator generally includes a quadrature signal generator 11 for generating a quadrature signal having I signal components and Q signal components, a first mixer 12 for mixing the I signal fout-I from the quadrature signal generator 11 with an input signal (RF signal), and a second mixer 13 for mixing the Q signal fout-Q from the quadrature signal generator 11 with the input signal (RF signal).
Such a quadrature signal generator should be designed, taking into consideration various characteristics such as frequency, phase, and amplitude characteristics, noise characteristics, bandwidth characteristics, power characteristics, and symmetric characteristics between I and Q signals. In this regard, a poly-phase filter, a frequency divider, or a ring type VCO (Voice Carry Over) is used for traditional quadrature signal generators. The characteristics of such devices will be described in brief hereinafter.
First, a description will be made of the poly-phase filter. This poly-phase filter is most frequently used as a quadrature signal generating device in designing an MMIC (Monolithic Microwave Integrated Circuit). The poly-phase filter has a structure improved in symmetry and repeatability over a traditional RC-CR filter structure mainly used to generate a Q signal. Since the traditional RC-Cr filter structure is sensitive to resistance and capacitance, it may exhibit considerable amplitude and phase errors when the resistance and capacitance lose a desired symmetry therebetween due to variations thereof.
Furthermore, the traditional RC-CR filter structure exhibits satisfactory amplitude and phase characteristics only in a narrow band. On the other hand, where poly-phase filters of multiple stages are connected in series, it is possible to obtain satisfactory phase and amplitude error characteristics in wide bands. In this case, however, there is a problem in that a signal loss of about 3 dB occurs for every additional stage even though such a stage addition achieves an expansion in applicable band. For this reason, in the case of a poly-phase filter designed to have multiple stages, there may be a degradation in noise characteristics and an increase in the signal loss of an associated oscillator. In order to compensate for such affects, the buffer circuit of the oscillator must have high voltage gain characteristics, thereby resulting in an increased consumption of current.
Now, the frequency divider will be described. Recently, demand for a multi-band (for example, a multi-band of 900 MHz and 1.8 GHz) has been increased. To meet such a demand, a direct frequency conversion has been required. Also, it has been required to eliminate use of external elements. In order to meet such requirements, an image rejection converter and a direct frequency conversion type receiver may be used. In this case, it is necessary to use a quadrature signal generator as a frequency divider. This quadrature signal generator is preferable because it operates at a frequency higher than that of frequency dividers of other types. The quadrature signal generator is operable at a high frequency in both the case of using a CMOS process and the case of using a bipolar process. In this case, however, it is important to determine an appropriate transistor size, taking into consideration operable frequency and power consumption because the operable frequency and power consumption are inversely related.
The most significant problem of such a frequency divider is that the input frequency is double the output frequency. In order to use such a frequency divider, it is necessary to generate a frequency higher than a usage frequency by two times. In this case, however, the associate frequency generator involves an increase in phase noise, and an increase in power consumption. Even when a frequency doubler is used, problems of an increase in phase noise and an increase in power consumption still occur.
The ring type VCO will now be described. This ring type VCO has a relatively small size because it has a wide oscillation frequency adjustment range while including no inductor. However, the ring type VOC is hardly used in an RF range because it exhibits considerably degraded phase noise characteristics, as compared to LC type VCOs. Besides, such a ring type VCO is mainly used as a frequency generator of 1 GHz or less because it can achieve an easy oscillation even in a high frequency range, and its wide frequency adjustment range provides an important advantage.
In order to solve the drawbacks involved with the above mentioned traditional quadrature signal generators, various quadrature signal generators have recently been researched and developed. An example of such conventional quadrature signal generators will be described hereinafter.
FIG. 2 is a schematic diagram illustrating the configuration of a conventional quadrature signal generator. The conventional quadrature signal generator shown in FIG. 2 includes an oscillator 21 for generating a differential oscillation frequency signal having differential oscillation frequency components, a frequency doubler 22 for doubling the frequency components of the differential oscillation frequency signal received from the oscillator 21, thereby outputting a signal having single-frequency components, a balun 23 for receiving the single-frequency signal from the frequency doubler 22, thereby outputting a signal having differential frequency components, and a frequency divider 24 for dividing the differential frequency signal received from the balun 23 by 2, thereby outputting a quadrature signal having quadrature frequency components.
In the conventional quadrature signal generator of FIG. 2, the output frequency of the frequency doubler is single. For this reason, it is necessary to use an additional balun in order to obtain differential frequencies from the single frequency.
Although the above mentioned conventional quadrature signal generator has an advantage in that it is possible to generate quadrature frequencies identical to the output frequency of the oscillator, there is a problem in that the complexity of the quadrature signal generator increases because an additional balun should be used. Furthermore, although the conventional quadrature signal generator can perform its function at a particular frequency, it is difficult to generate perfect differential frequencies in a frequency range beyond the particular frequency because a frequency delay occurs due to the characteristics of the balun. Where it is impossible to generate such perfect differential frequencies, there is a problem in that mismatch of I and Q signals occurs.